One way to improve efficiency and stall margin of axial-flow turbines and compressors is by means of variable guide vanes which control blade incidence angle. Although the most commonly used type of variable guide vanes are stiff (relatively inflexible), whether rigidly mounted or axially rotatable, there does not appear to be any fully flexible variable guide vanes which are sufficiently beneficial so as to find their way into common use.
A hollow flexible vane with inner working parts, responsive to rotation of a trunion or rod, is disclosed in U.S. Pat. No. 3,237,918. A vane, comprised of a thin flexible material, is distorted into a desired shape in response to stressing one portion by movement thereof, while other portions remain rigidly attached to static structure, is disclosed in U.S. Pat. No. 3,723,021. A simple hinge which rotates the trailing edge of a vane is illustrated in U.S. Pat. No. 4,741,665. The foregoing disclosures appear to be unduly complex or difficult to produce.
A simpler design, in U.S. Pat. No. 7,942,632, includes stiff material, or compliant material such as silicone rubber, fixed to an arm at the bottom of the trailing edge of an airfoil, the arm being rotatable through a rod by a conventional unison ring disposed for slight rotation about the axis of a low bypass ratio, turbofan engine. The alteration of the airfoil shape in that case may be too limited to satisfy some utilization needs.